Vulcanization is a chemical process which improves the bulk properties of rubber for use in many applications. Vulcanized rubber is used in vast quantities in a myriad of products including automobile tires, shoe soles, and hoses. However, recycling vulcanized rubber to obtain useful materials is often a difficult and time consuming process.
The problem of efficiently recycling vulcanized rubber to produce raw materials suitable for reuse in manufacturing or other applications has proven to be difficult to solve. The process of recycling this material typically requires a devulcanization which, as the name suggests, attempts with varying degrees of success to reverse the effects of vulcanization.
Once devulcanized, the material may be further broken down into useful subcomponents such as carbon black and polymers such as polyisoprene, polybutadiene, styrene-butadiene, isoprene-butadiene, styrene-isoprene, styrene-isoprene-butadiene, butadiene acrylonitrile, chloro isobutylene-isoprene, and bromo isobutylene-isoprene. The above list of polymers is not intended to be limiting. The present invention addresses the problem of separating the carbon black component from the polymer component of devulcanized rubber material.
One way to achieve a physical separation between carbon black and polymer in devulcanized material is to dissolve the polymer in a solvent such as toluene. The resulting mixture is typically a suspension of carbon black in a solvent/polymer solution. The problem of separating the carbon black from the polymer becomes one of removing these suspended particles from the solution.
The processing of devulcanized scrap rubber with the use of a solvent is well known in the art. Prior processes have been described where ‘moderate sized chunks’ of scrap rubber with diameters of up to 6 inches are dissolved in aromatic oil in a stirred reactor under particular pressure and temperature conditions. It is apparent that carbon black particles are distributed throughout the resulting solution in such a ‘stirred reactor’.
Centrifuge technology has been used in many previous applications to separate suspended solids from a fluid. The design of a centrifuge required to achieve a desired degree of separation depends on many factors including the type of centrifuge, the properties of the particles to be separated, and the properties of the surrounding fluid. Carbon black particles used in the reinforcement of rubber tend to be quite small compared to the size of particles that typical industrial centrifuges are designed to capture. In practice it is not a simple matter to separate the small size carbon black particles from the polymer/solvent solution in a cost effective manner, with centrifuge technology.
Another common method used to remove suspended solids from a solution is filtration. The use of filters is also not ideal in the present application. Filters that are fine enough to be suitable for the filtration of carbon black also tend to result in a slow separation process if gravity is employed to push the solution through the filter. If pressure or a vacuum is applied to increase the rate of filtration, then carbon black may be pushed through such filters. Another problem with filtration is blinding wherein the filtration rate is dramatically reduced by a build-up of material on the filter. This problem is exacerbated by increasing polymer molecular weight and the presence of both carbon black and polymer in the mixture to be filtered. The polymer tends to become bound-up with carbon black deposited on the filter creating an impenetrable layer.
Pre-coating the filter is a method that attempts to improve the efficiency of filtration by pre-treating the filter surface with a layer of the material to be filtered. However, in removing the small carbon black particles that are suspended in the polymer/solvent solution, the cake formed on the filter for the pre-coating can rapidly lead to binding of the polymer/solvent solution and carbon black.
Emulsification techniques whereby a slurry of suspended carbon black particles in a solution of polymer and solvent is mixed with an immiscible fluid such as water have also proven to be ineffective for the purpose of separating carbon black into a separate phase from a polymer/solvent solution.
A need exists for an improved method of separating carbon black from the polymer of devulcanized rubber that is both cost efficient and effective in separating these components.